The use of the now well-known hot cathode deuterium arc lamp (D.sub.2 lamp) as a near UV source for a spectrophotometer is common. This lamp requires four hundred volts or more for the arc to strike, yet when running the arc current must be kept very constant, typically at 300 mA, with an arc voltage drop of about 70 to 90 V. In addition, to strike the arc, additional power, for instance 1 A. at 10 V., must be supplied to the cathode heater to raise the cathode temperature enough to establish an adequate arc plasma. The arc, once struck, will keep the cathode hot, hence, this external power should then be switched off to prevent the cathode from overheating.
All these operating power requirements have in the past been supplied by special power supplies. A typical supply might be built up of a high voltage supply, a running voltage supply, a low voltage high current supply and various relays, timers and regulating circuits to control these supplies in the proper sequence and manner. Conventionally, the regulating circuit for anode current has been of the analog type, utilizing a series pass transistor for control. Such a circuit normally has low efficiency because of the resistance loss in the series control transistor. This loss appears as heat which must be dissipated in a heat sink of substantial size requiring good ventilation, an arrangement necessitating a fairly bulky supply structure. The size, cost and losses of these combinations places high demands on the equipment group for the spectrophotometer.
The development of a new, low cost, photodiode array spectrophotometer set up a demand for a drastically smaller, higher efficiency regulated power supply for the D.sub.2 lamp source.
One of the prior problems was associated with the traditional requirement of heating the D.sub.2 lamp cathode before applying the high starting voltage to the anode. This was usually effected by using a timing circuit or relay for switching on power to the cathode heater so that the cathode reaches a red heat before the starting voltage is applied. The philosophy of this prior art has been, that applying starting voltage to an unheated cathode would shorten the lamp life by erosion of the cathode emitting layer. In most cases timing circuits or relays also were used to switch off the heater current after the lamp was running, since the arc drop was capable of keeping the cathode hot. Such timing device were costly and bulky.
Unexpectedly, we have found the prior art concept of preheating the cathode to be not only unnecessary but also inadvisable. It can be shown that applying starter voltage to a cold cathode before the lamp has started will not damage the cathode since no current is flowing. Research at M.I.T., resulting in the invention of the dispenser cathode by E. A. Coomes, showed that deterioration of the cathode emitting material is due primarily to heavy arc current. If the cathode is heated without arc current flowing, the arc will strike when the temperature reaches a value high enough to supply sufficient ions for the arc, and no deterioration of the cathode will result. Once the arc strikes the heater current can be switched off to prevent the cathode from being overheated. According to the present invention, this starting sequence is effected through solid state sensor and switching means without timers or relays, thus reducing sharply both the cost and the size of the circuitry.
The present application is directed to a novel switching type power supply, which successfully accomplishes the objectives set forth.
It is an object of this invention to provide power supply. means driven by a low dc voltage input.
It is a further object that said supply means generate a low current, high voltage output adequate to start a D.sub.2 lamp when its cathode is hot.
It is a further object that said supply means also generate a closely regulated medium current output of proper value to run said D.sub.2 lamp.
It is yet a further object that said supply means deliver a high current, low voltage to heat the cathode of said D.sub.2 lamp subsequent to the application of the starting voltage, this heater current being terminated when the lamp has started and reached self-heating stability.
It is also an objective that all components of this supply means be solid state and multi-functional insofar as practical to minimize number, size and cost.
It is also an objective that this supply means be switchable on or off by a TTL or equivalent command.
It is also an objective that all components be mountable on a single small printed circuit board.
Other objects and advantages will become apparent from the following detailed description and accompanying drawings.